Engines may be configured with an exhaust heat recovery system for recovering heat of exhaust gas generated at an internal combustion engine. The heat is transferred from the hot exhaust gas to a coolant through an exhaust gas heat exchanger system. The heat from the coolant, circulated through an exhaust gas heat exchanger, may be utilized for functions such as heating the cylinder head, and warming the passenger cabin, thereby improving engine efficiency. In hybrid electric vehicles, the recovery of exhaust heat improves fuel economy by enabling engine temperatures to be maintained longer, thereby allowing for a faster engine shut-off and extended use of the vehicle in an electric mode.
Exhaust heat may also be retrieved at an exhaust gas recirculation (EGR) cooler. An EGR cooler may be coupled to an EGR delivery system to bring down the temperature of recirculated exhaust gas before it is delivered to the intake manifold. EGR may be used to reduce exhaust NOx emissions. Further, EGR may be used to assist in the reduction of throttling losses at low loads, and to improve knock tolerance.
Various approaches are provided for exhaust heat recovery and EGR cooling. In one example, as shown in US 20120260897, Hayman et al. discloses an engine system wherein exhaust gas from a first group of cylinders is directed to an exhaust tailpipe. Exhaust gas from a second group of cylinders is either directed to an intake manifold via an EGR passage. In addition, based on the EGR requirement of the engine, a portion of the exhaust from the second group of cylinders may be diverted to the exhaust tailpipe via a bypass passage. An EGR cooler coupled to the EGR passage is used to cool the exhaust gas prior to mixing of the EGR with combustion air and entry of the mixture into the intake manifold.
However, the inventors herein have recognized potential disadvantages with the above approaches. As one example, it may be difficult to coordinate EGR cooling with exhaust heat recovery. In particular, the heat recovered at the EGR cooler cannot be used for heating a cabin space. As a result, a distinct heat exchanger is required for cabin heating. Likewise, even if heat is extracted from exhaust gas at a heat exchanger, the cooled exhaust gas is not recirculated, resulting in the need for a distinct EGR cooler. The additional components add cost and complexity.
The inventors herein have identified an approach by which the issues described above may be at least partly addressed. One example method comprises recirculating exhaust gas to an engine intake by flowing a first amount of exhaust gas in a first direction through a first section of a heat exchanger, and flowing a second amount of exhaust gas in a second, opposite direction through a second section of the heat exchanger.
In one example, an engine system may be configured with a heat exchanger positioned downstream of a catalytic convertor in an exhaust bypass disposed parallel to a main exhaust passage. The heat exchanger may be asymmetrically designed with two separate cooling sections having an EGR passage coupled to the intermediate section of the two cooler sections. A single butterfly valve may be used to enable exhaust gas to be diverted into the bypass passage, and through the heat exchanger in one of two directions, a position of the valve adjusted based on engine operating parameters. Depending on engine requirement, EGR may be cooled to different levels. In one example, during a first operating mode, when required EGR temperature is above a threshold, the valve position may be adjusted such that the exhaust gas may flow through the first section of the two section EGR cooler before being delivered to the engine manifold. In another example, during a second operating mode, when required EGR temperature below the threshold, the valve may be adjusted to flow the exhaust gas in opposite directions through both sections of the EGR cooler before entering the engine intake manifold. During the flow in both first and second operating mode, exhaust heat is transferred to a coolant circulating through the heat exchanger. The warmed coolant may be circulated back to the engine (such as when engine heating is required) and/or utilized for heating a passenger cabin of the vehicle (such as when cabin heating is requested). Alternatively, the extracted heat is transferred to the radiator for dissipation into the atmosphere.
In this way, the heating requirements of an engine system may be met using a single heat exchanger. By providing the functions of an EGR cooler and an exhaust gas heat exchanger via a single heat exchanger, cost and component reduction benefits are achieved without limiting the functionality or capability of either system. In addition, the specific configuration of a single heat exchanger in a bypass exhaust passage allows for a shorter EGR passage length, which reduces EGR transport delays. The technical effect of using a butterfly valve to regulate the passage of exhaust gas through the bypass passage is that exhaust gas can be flowed in both directions across the heat exchanger. As such, this improves the heat transfer efficiency and reduces the requirement for long EGR passages. By using a single butterfly valve to control the EGR flow, number of components in the system is reduced which facilitates quicker warm-up of the coolant due to less thermal mass. By using a split EGR cooler it is possible to regulate the degree of cooling of the EGR depending on engine requirements. The separate cooler and heat recovery systems on being combined to one, reduces manufacturing cost. Since the EGR cooler is located downstream from the catalyst convertor, catalyst functionality is not compromised. Overall, by improving the amount of waste heat that can be recovered from exhaust using fewer components, engine fuel economy and performance is improved.
It should be understood that the summary above is provided to introduce in simplified form a selection of concepts that are further described in the detailed description. It is not meant to identify key or essential features of the claimed subject matter, the scope of which is defined uniquely by the claims that follow the detailed description. Furthermore, the claimed subject matter is not limited to implementations that solve any disadvantages noted above or in any part of this disclosure.